The invention relates to an optoelectronic sensor with overflow arrangement which contains a series of sensor elements arranged on a doped semiconductor body of the first conductivity type and comprising MIS capacitors for the collection of optically generated minority carriers. The sensor elements are separated from one another by means for the generation of a potential threshold for the minority carriers in the semiconductor body. The sensor further contains at least one shift register formed of further MIS capacitors on each side of the sensor row and contains transfer electrodes arranged to the side of the sensor electrodes which alternately connect the sensor elements with a capacitor of the two shift channels according to the principle of charge-coupling.
One-dimensional optoelectronic sensors contain sensor elements arranged in a row which are in a position to convert the intensity of the light incident upon them into an electric signal. They are employed, for example, for scanning the linear dimensions of objects, for example, work pieces in production, or also for the line-wise scanning of images to be electronically read, for example, in facsimile transmission.
In increasing amount, metal electrode isolator semiconductor (MIS) capacitors are being used as sensor elements and which consist of a doped semiconductor body covered with an insulation layer and a light-permeable, superimposed "metal electrode" (generally consisting of strongly doped polysilicon). When, for example, in a p doped semiconductor substrate, the substrate connection of the MIS capacitor is negatively biased with respect to the electrode, then the (positive) majority carriers of the substrate are stripped from the substrate connection, whereas the optically generated minority carriers are held under the sensor electrode. This charge package consisting of minority carriers can be periodically read out into a shift register formed of further MIS capacitors in that a transmission electrode (transfer gate) is arranged above the insulation layer between the sensor electrode and the electrode of a shift register capacitor, which transfer gate, insulated only by a thin oxide layer, connects directly to or, respectively, slightly overlaps the two electrodes. When the transfer gate is applied to a negative potential with respect to the substrate connection, then a potential barrier arises for the optically generated, negative charge carriers (minority carriers) which prevents the flow of the minority carriers out of the sensor element into the shift register during the collection of the optically generated charge carriers. When, however, the transfer gate is applied to a sufficiently positive potential, then a potential difference arises for the minority carriers from the sensor element into the semiconductor area under the transfer gate and the charge flows off in that direction. In the same manner, the charge carriers can then be further displaced into the capacitor of the shift register.
For a reading of the image which is as faithful as possible, it is necessary to lay a plurality of sensor elements which are as close as possible to one another. Therefore, a minimum distance between two sensor elements of a one-dimensional sensor is predetermined in that a plurality of further MIS capacitors for the displacement of the charge are required in the shift register proceeding next to the sensor row between two capacitors designed for the reception of the charge packets from the sensor elements. In the article "Modulation Transfer Function of Quadrilinear C.C.D. Imager" (Electronics Letters, Vol. 12, No. 25), Herbst and Pfleiderer specify a particularly spacesaving arrangement in which a shift register is arranged on each side of the sensor row. The sensor elements are now coupled in succession once with an MIS capacitor of the right and once with an MIS capacitor of the left shift register via transmission electrodes. It is further proposed to arrange a second, parallel shift register on each side. Each second charge package which is displaced into the inner shift register in the immediate proximity of the sensor row via a first transmission electrode or transfer gate is then further displaced by means of a second transmission electrode or transfer gate into an MIS capacitor of the corresponding outer shift register which runs parallel. The MIS capacitor of the inner shift register required for the displacement of the charge package out of the sensor element into the outer shift register can be used, upon the subsequent read-out of the charge package out of the shift registers, for the displacement of the charges along the shift register.
With strong light incidence upon individual sensor elements or a group of sensor elements, it can occur that so many minority carriers are optically generated that they overflow into neighboring sensor elements (blooming) and lead to the destruction of the information in larger areas of the sensor. This phenomenon can be prevented when oppositely doped overflow areas (drain areas) are provided on the semiconductor surface which are contacted with corresponding overflow electrodes and can strip charge carriers overflowing out of the sensor elements out of the substrate. Between the sensor elements and the overflow areas, however, a potential threshold must exist which is high enough that the minority carriers are prevented from flowing into the overflow area upon normal exposure and only diffuse off into the overflow area upon exposure that is too strong, i.e. when all minority carriers can no longer be stored in the sensor element. This potential threshold must be lower than the potential threshold between the individual sensor elements and can be generated in that an additional gate is arranged between the overflow areas and the sensor elements by means of which the potential path between overflow area and sensor electrode can be controlled. In order to apply this principle to one-dimensional sensors, however, considerable space is required for the overflow areas, overflow electrodes and their control installations. This considerably limits the resolution of the sensor.